APS_April 2023

J ournal of the A merican P omological S ociety

72

Table 2. Severity and incidence of fire blight response in 20 diverse Pyrus accessions. Table 2. Severity and incidence of fire blight response in 20 diverse Pyrus accessions. 385 Experiment A Experiment B

MS groups y

Ave. %SLB x

Max %SLB v

% Incid. u

Num. indiv. t

MS groups

Ave. %SLB

Max %SLB

% Incid.

Num. indiv.

Accession z

Du Li

a

100.0 100.0 100.0

19 20 20 20 19 20 20 20 20 17 20 11 16 17 20 20 20 20 20 20

a

94.7 100.0 94.7

19

Hybrid 4

ab

76.7 100.0 67.0 100.0

95.0 90.0

ab ab

84.7 100.0 100.0 18

P. salicifolia ( hybrid ) - Russia

b b

85.5 100.0 95.0

20

Bartlett

62.0 100.0 100.0 35.2 100.0 100.0 59.9 100.0 100.0

abc 85.4 100.0 100.0 20

Mustafabey GE-2004-131 OH×F 333 B

bc bc

bcd 59.6 100.0 85.0

20

bcde 65.8 100.0 100.0 20

cdef 44.6 100.0 72.2 defg 33.5 100.0 40.0 defg 25.0 100.0 68.4

18 20 19

Hybrid 1 Hybrid 2

cd cd de de de de de de de de de de de

26.8 100.0 22.9 100.0 14.6 100.0

40.0 60.0 35.0 64.7 45.0 63.6 31.3 29.4 30.0 20.0 55.0 25.0 20.0

OH×F 333 A

P-87

4.0 8.0 4.1 3.1 7.5 8.7 1.2 0.9 1.1 0.1

12.0 71.5 30.7 19.4 86.9

defg 16.3 76.7 89.5

19 20

P. xerophila – Lawyer Nursery

efg fgh

27.4 100.0 60.0 10.6 31.7 44.4 7.0 63.8 33.3 2.9 21.2 37.5

Hybrid 5

9

Farmingdale

gh gh gh gh gh gh gh

15 16 19 20 20 20 17

OSU-8

Anjou (Beurre d’Anjou)

10.9 100.0

1.4

9.5

26.3

Old Home Hybrid 3 OH×F 87

87.7

2.2 40.6 20.0

3.9 6.7

1.7

7.6

50.0

2.6 16.7 50.0 1.5 12.5 23.5

OSU-2

10.6

z Accessions were ordered within their respective mean separation groups for p ercent shoot length blighted (%SLB) for easy comparison between experiments. Accession ‘OH×F 333’ was unable to be aligned across experiments and is designated with an ‘A’ or ‘B’ superscript to indicate the respective experiment. y Mean separation groups within experiments were determined using an analysis of variance and Tukey’s Honest Significant Dif ference test for %SLB. x Average of individuals’ %SLB within accession, calculated by dividing shoot length blighted by total shoot length, multiplied by 100. v Maximum %SLB represents the most severe response for an individual within each accession u %Incidence is calculated using the number of trees with fire blight response divided by total number of individuals per accession , multiplied by 100. t Number of individuals inoculated per accession. 20 z Accessions were ordered within their respective mean separation groups for percent shoot length blighted (%SLB) for easy 386 comparison between experiments. Accession ‘OH×F 333’ was unable to be aligned across experiments and is designated 387 with an ‘A’ or ‘B’ superscript to indicate the respective experiment. 388 y Mean separation groups within experiments were determined using an analysis of variance and Tukey’s Honest Significant 389 Diff renc test f r %SLB. 390 x Averag of individuals’ %SLB within accession, calculated by dividing shoot length blighted by total shoot length, multiplied 391 by 100. 392 v Maximum %SLB represents the most severe response for an individual within each accession 393 u %Incidence is calculated using the number of trees with fire blight response divided by total number of individuals per 394 accession, multiplied by 100. 395 t Number of individuals inoculated per accession. Hybrid 6 ( P . betulifolia -1 × P-79) e 1.4 5.0 h 0.8 15.4 5.0

396 397 398

Ideally, a rootstock would have both low disease severity and incidence, such as ‘Hy brid 6’ (Table 2). ‘Hybrid 3’ maintained low severity (%SLB and Max %SLB), but had moderate incidence. A rootstock that sus tains mild infections has a greater chance of surviving and can be re-grafted if neces sary. However, high incidence of rootstock infection can potentially lead to an increased number of susceptible scions at risk for bac terial transmission (Santander et al., 2020). ‘Hybrid 5’ and ‘P-87’ had lower %SLB with a moderate Max %SLB and moderate inci dence of infection. When an accession has

moderate incidence with high severity, fewer trees may be infected, although the infection is more likely to lead to tree loss. In summary, this fire blight study pro vides comparative data for potential breed ing parents evaluated with the same strain and similar greenhouse growing conditions. Six accessions were identified as highly sus ceptible, and new information is reported for nine accessions with previously unknown fire blight responses. Accessions were iden tified in this study that had comparable or lower susceptibility than the industry stan dard ‘OH×F 87’. The nine accessions that ex -